Power supply controller and method therefor

1. A battery charger comprising:

a switching controller configured to form a switching control signal suitable to control a power switch to control current through a primary inductor of a transformer;

a pseudo generator circuit configured to receive an auxilary voltage from an auxiliary winding of the transformer and generate a current sense signal that is representative of current flow through a secondary winding of the transformer;

a sampling circuit configured to sample the auxiliary voltage at a substantially zero crossing of the current sense signal and form a sampled signal responsively to the current sense signal;

an error amplifier coupled to receive the sampled signal and form an error signal wherein the switching controller uses the error signal to form the switching control signal and wherein the pseudo generator circuit is copied to receive the error signal and store a first signal that is representative of the error signal, and configured to decay the first signal at a first rate that is representative of a decay rate of the current flow through the secondary winding; and

a current limit circuit configured to receive the current sense signal and terminate an on-cycle of the switch responsively to a first value of the current sense signal.

2. The battery charger of claim 1 wherein the switching controller and the auxiliary voltage have a common reference.

3. The battery charger of claim 1 wherein the pseudo generator circuit stores a feedback signal that is representative of the output voltage on first a storage capacitor, charges a second capacitor to a value representative of the error signal stored. on the first capacitor, and discharges the second capacitor at the first rate.

4. The battery charger of claim 1 further including a sampling circuit configured to form a first stored signal that is representative of the error signal from an error amplifier, sample the first stored signal and form a second stored signal that is representative of the first stored signal, and a decay the second stored signal at a rate that is representative of the decay rate of the current flow through the secondary winding.

5. The battery charger of claim 1 wherein the current limit circuit includes an integrator operably coupled to integrate the current sense signal and to terminate the on-cycle of the switching control signal responsively to the first value.

6. The battery charger of claim 5 further including a comparator configured to compare a result of the integration of the current sense signal to a reference value.

7. A method of forming a power supply controller comprising:

configuring a switching controller to form a switching control signal suitable to control a power switch to control current through a primary inductor of a transformer to regulate an output voltage on a secondary winding of the transformer;

configuring the power supply controller to receive a sense signal that is representative of an auxiliary voltage from an auxiliary winding of the transformer and form a current sense signal that is representative of current flow through the secondary winding including configuring the power supply controller to store an error signal from an error amplifier to form a stored value and to decay the stored value at a rate that is representative of a decay rate of the current flow through the secondary winding; and

configuring a current limit circuit to receive the current sense signal and cause the power switch to be disabled responsively to a first value of the current sense signal.

8. The method of claim 7 wherein configuring the current limit circuit to receive the current sense signal includes operably coupling the current sense signal to form an average value of the current sense signal and compare the average value to a reference value.

9. The method of claim 7 further including configuring the power supply controller to receive a feedback signal that is representative of the output voltage and use the feedback signal to form the current sense signal.

10. The method of claim 9 wherein configuring the current limit circuit to receive the current sense signal includes operably coupling the current sense signal to form an average value of the current sense signal and compare the average value to a reference value.

11. A method of forming a power supply controller comprising:

configuring a switching controller to form a switching control signal suitable to control a power switch to control current through a primary inductor of a transformer to regulate an output voltage on a secondary winding of the transformer;

configuring the power supply controller to use an auxiliary voltage from an auxiliary winding of the transformer to form a current sense signal having a zero crossing that is substantially the same as a zero crossing of a current through the secondary winding;

configuring the power supply controller to store a value of the auxiliary voltage at the zero crossing as a feedback signal that is representative of a value of the output voltage;

configuring the power supply controller to use the current sense signal to limit a value of the current through the secondary winding to no greater than a first value.

12. The method of claim 11 wherein. configuring the power supply controller to use the auxiliary voltage includes configuring the switching controller to be referenced to a common voltage to which the auxiliary winding is referenced.

13. The method of claim 11 further including configuring an error amplifier to use the auxiliary voltage from the auxiliary winding to form an error signal that is representing a deviation of the output voltage from a desired value of the output voltage, and configuring the power supply controller to use the error signal to form the current sense signal.

14. The method of claim 11 further including configuring the power supply controller to receive a feedback signal that is representative of the output voltage, and configuring the power supply controller to use the feedback signal to form the current sense signal.

15. The method of claim 11 wherein configuring the power supply controller to use the current sense signal includes operably coupling a current limit circuit to receive the current sense signal, form a signal that is representative of an average value of the current sense signal, and compare the average value to a signal that is representative of the first value.